The developer that acquired 51 Sleeper St. in Boston’s Fort Point last year has unveiled plans to rebrand the 152,000-square-foot office building as 3 Seaport and add a new lobby and possible roof deck.

Brookfield Properties said Boston-based Merge Architects is designing the renovations and additions as it seeks tenants for a pair of 20,100-square-foot floor plates with two more potentially becoming available in 2020.

Brookfield bought the property  from TIAA-CREF in December for $91 million.

Cushman & Wakefield represents ownership in leasing. Existing tenants include Bergmeyer, United Way, The U.S. General Services Administration and The Department of Early Education and Care. Hopsters Brewing Co. occupies the retail space on the ground floor.